The effect of triacyl-sn-glycerol structure on the metabolism of chylomicrons and triacylglycerol-rich emulsions in the rat.

A systematic study was undertaken to observe the effects of dietary (dioleoyl) triacyl-sn-glycerol structure on chylomicron composition and metabolism. First studied was a series of 1,2-dioleoyl-3-(saturated)acyl-sn-glycerols, where the fatty acid esterified at the 3-position was varied from 14 to 24 carbons. Next a series of 1,3-dioleoyl-2-acyl glycerols was studied, with various fatty acids esterified at the glycerol 2-position. These stereospecific triacyl-sn-glycerols were fed to donor rats and lymph chylomicrons were isolated, analyzed, and reinjected into recipient rats to study their disappearance from plasma and delivery to tissues. As shown by their compositions, chylomicrons obtained after feeding triacylglycerols containing all sn-3 fatty acid of chain length greater than 20 carbons were under-represented, possibly due to poorer digestion by lipases, or poorer absorption by the intestine. The 18-carbon saturated chain fatty acid (stearic acid) was equally well represented in chylomicrons whether in the 2- or 3-position of the fed triacylglycerol. The presence of increased amounts of long-chain saturated fatty acids in donor chylomicron triacylglycerols affected the metabolism of chylomicrons injected into the bloodstream of recipient rats. In particular the rate of removal of labeled cholesteryl esters, tracing removal of the partially degraded chylomicron remnants was slowed by the saturated chains, with palmitic acid and the 20-carbon fatty acid, arachidic acid, showing the most severe effects. There were clear differences in the removal from plasma of injected lymph chylomicrons derived from fed triacylglycerols containing stearic acid in either the 2- or 3-position, with evidence for remnants from the symmetrical triacylglycerols being less rapidly removed from the circulating blood. This effect was investigated further by injected model emulsions of chylomicrons, where the 2-position was substituted with saturated or transunsaturated acyl chains. Quantitation of removal from the blood stream of these model lipoproteins confirmed that a saturated or transunsaturated long chain fatty acid at the 2-position of the emulsion triacylglycerols slowed remnant removal from the blood. In some cases, with both lymph chylomicron and with emulsions, the lipolytic step mediated by lipoprotein lipase was also slowed.